Size Effects in Nanocrystalline Thoria

Abstract

The facile chemical precipitation method and subsequent thermal treatment were shown to be suitable for preparation of crystalline ThO2 nanoparticles (NPs) in a wide range of particle sizes (from 2.5 to 34.3 nm). The obtained NPs were investigated with X-ray diffraction, high-resolution transmission electron microscopy and X-ray absorption techniques to find out the possible size effects associated with nanocrystalline thoria. For 2.5 nm NPs, the lattice parameter of ThO2 was found to increase by up to 1.1 %, in comparison with the bulk material. The decrease in the particle size was also accompanied by a significant decrease in the Th-Th coordination number.

Probing the local structure of nanoscale actinide oxides: a comparison between PuO2 and ThO2 nanoparticles rules out PuO2+x hypothesis

“…Carbonate and hydroxyl surface groups have been suggested to exhibit a tensile effect on the crystalline lattice. 35 Generally, the lower nanoparticle sizes observed for PuO 2 samples compared to ThO 2 when using similar ring conditions can be attributed to the different mechanisms and temperatures related to the decomposition of oxalate precursors for both the oxides. 52,57 The nanostructure of the oxides obtained by the oxalate route appears to be controlled by the ring temperature and its duration, which involves a loss of nanoscale organization when they both increase.…”

“…54 The resulting unit cell volume may compress or expand as a function of the material studied. 25,35,54 For instance, an increase in the lattice parameter has been observed for CeO 2 nanoparticles smaller than 20 nm. 55,56 The lattice parameter of a 0 ¼ 5.600(6)Å has been reported for 14 nm ThO 2 particles; 52 Bouexiere et al reported the lattice parameter a 0 of 5.4042(2)Å for 3.7 AE 1.0 nm PuO 2 nanoparticles prepared by hydrothermal 48 Recently, the expansion of the lattice parameter has been observed for ThO 2 nanoparticles that exhibit different sizes.…”

“…These two peaks are respectively assigned to the An-O and An-An coordination spheres in the AnO 2 uorite crystal system. 11,[16][17][18]27,33,35 A strong decrease in the intensity of both An-O and An-An shell peaks is noticed on decreasing the ring temperature for both ThO 2 and PuO 2 samples. This modication in the local actinide structure can be ascribed to the decrease in crystallinity of the sample, which affects the local order in bulk materials (increase of DWF), or to the particle size variation, which signicantly affects the coordination number in the nanometer range.…”

“…XANES and EXAFS spectroscopic studies are known to be sensitive to size effects and the reduced number of neighbouring atoms at the surface of a particle in comparison to the bulk, which becomes signicant for the smallest particles. 23,26,35,[62][63][64] As an instance, previous calculations showed that ca. 27% of Pu atoms are present at the surface of a PuO 2 particle of 2.9 nm in diameter.…”

“…32 The reported EXAFS spectra for ThO 2 samples already show similar Fourier transforms compared to the An-O and An-An coordination spheres. 30,[33][34][35] In addition, Th only exists in the (+IV) oxidation state and therefore, allows the comparison of Pu compounds so as to avoid misinterpretations related to the potential contribution of other oxidation states in the crystalline structure. This paper rst reports the preparation and the characterization of AnO 2 nanoparticles (An ¼ Pu or Th) in the form of suspended colloids, nanopowders, and nanostructured oxides.…”